1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image on a recording medium by using an electrophotographic printing method or the like and to a image density control method therefor.
2. Description of the Related Art
In recent years, there has been an increasing demand for improving the image quality and stability of image forming apparatuses. As a result of an effort to improve the image quality, in image forming apparatuses, a plurality of image forming modes have been provided. In such image forming apparatuses, an image is formed in a mode suitable for characters or in a mode suitable for photographs by changing lines, dots, or the definition (the number of lines per inch) of the image to be formed. Expressions of images are thus enriched to meet various user needs in the market.
One of consideration items to which prime importance is attached in a stability improving scheme is density gradation stability. In particular, output densities of an image forming apparatus vary due to environmental changes, changes with time and the like. There is, therefore, a demand for the provision of a density control sequence for maintaining correct density gradations of the image at all times.
In conventional image forming apparatuses, therefore, correction parameters (e.g., a lookup table: LUT) are set such that proper relationships are maintained between input image densities and output image densities. Input image data is corrected on the basis of a LUT to maintain proper relationships between input image densities and output image densities.
The LUT is obtained, for example, by reading the optical density of an image pattern formed on a recording medium such as recording paper or an image pattern formed on an image bearing member such as a photosensitive drum. In a case where a high-quality output image is required, it is desirable to perform density gradation control using a recording medium. Also, it is desirable to perform density gradation control with respect to each of a plurality of image processing modes in order to obtain high-quality images in the plurality of image processing modes.
An image forming apparatus which performs gradation correction without forming gradation patterns on an image bearing member in all image processing modes (see Japanese Laid-Open Patent Publication (Kokai) No. 08-156330) has been proposed. This image forming apparatus functionizes the correlations between input/output characteristics (γ-characteristics) for the image processing modes or patternizes the correlations. Then, on the basis of a gradation pattern formed on the image bearing member in one of the image processing modes and the functionized or patternized correlations, the image forming apparatus performs density gradation control uniquely in another of the image processing modes.
As described above, the image forming apparatus described in Japanese Laid-Open Patent Publication (Kokai) No. 08-156330 is disclosed with respect to gradation control using an image bearing member. However, a method of comparing the density of a gradation pattern actually printed on a recording medium with the desired density and performing gradation control on the basis of the comparison result enables gradation control with higher accuracy than the method described in Japanese Laid-Open Patent Publication (Kokai) No. 08-156330.
This density gradation control using a recording medium is performed by a user moving the recording medium output from the image forming apparatus to a reader and making the reader read the recording medium at the time of execution of density gradation control. However, a heavy burden would be put on the user if density gradation control using a recording medium needs to be performed with respect to all the plurality of image processing modes that the user may use.